Temperature sensors are used in the semiconductor industry to monitor the operating temperatures of devices fabricated on integrated circuit chips. Typically, the actual sensor and the associated sensing circuitry are fabricated on a separate chip from that which contains the device whose temperature is being monitored. The sensor chip is then placed close to the chip containing the device of interest. This means that in reality, the temperature sensor is measuring the temperature of the local environment of the device, not its actual temperature.
Semiconductor junction diodes are often used as temperature sensors for integrated circuit devices. Such a diode sensor is shown in FIG. 1. When such a diode is biased at a given current and the junction temperature varies, the voltage across the diode shows an almost linear variation with temperature. The diode voltage versus temperature curve has a negative coefficient, with the diode voltage equal to the bandgap voltage of silicon at zero absolute temperature (0.degree. Kelvin). The relationship between V.sub.d (Tr), the diode voltage at a reference temperature Tr, and V.sub.d (T), the diode voltage at a temperature T can be modeled as: ##EQU1##
This is a linearized diode equation representing the V.sub.d versus T relation. In the equation, V.sub.bg is the silicon bandgap voltage of the diode. As is evident, measurements of the diode voltage can be used directly for temperature sensing. However, the V.sub.d versus T relationship has a curvature (non-linearity) of the temperature-to-forward voltage transfer function which is on the order of a couple of percent over the normal silicon operating temperature range. Such sensors also show variation from process to process and across the silicon wafer (i.e., sensor-to-sensor). The junction diodes thus require calibration and introduce errors when used as a temperature sensor.
The diode forward voltage as a function of temperature may also be expressed in terms of the current source, (I.sub.F), and the reverse saturation current (I.sub.S) In this case the diode voltage (V.sub.F) is expressed as: EQU V.sub.F =(kT/q) ln (I.sub.F /I.sub.S), (2)
where k is Boltzmann's constant and q is the electron charge.
As noted, using a single diode as a temperature sensor has the disadvantage that the measurements are subject to process variation and non-linearity problems. For this reason a differential, two sensor measurement technique is sometimes used.
A differential technique is based on measuring the difference in the forward voltages of two junctions operating at different current densities. FIG. 2 is a schematic showing how two junction diode temperature sensors may be used as part of a differential temperature measurement technique. Different current densities can be achieved with various combinations of diode areas and currents. A typical approach might be to give diodes D1 and D2 equal collector currents, but different areas (for example, D2 might be ten times the size of D1). The difference between their forward voltages will then be proportional to the log of the current density ratio and to absolute temperature, as can be seen in the following equation: EQU V.sub.F1 -V.sub.F2 =(kT/q) ln (J.sub.1 /J.sub.2). (3)
In the equation, J.sub.1 and J.sub.2 are the current densities (current per unit area) in D1 and D2.
The differential output voltage is a fairly small voltage (typically only a fraction of a millivolt/K), so it is amplified to create a more convenient temperature coefficient (such as 10 mV/K) at the processing circuitry output. This technique is used to produce silicon temperature sensors with output voltages or currents proportional to absolute temperature.
FIG. 3 is a diagram showing a temperature sensor 100 formed from a junction diode 102 which may be used to sense the temperature of an integrated circuit. Junction diode 102 is characterized by its junction temperature, T.sub.j. The junction is formed from a p-type region which is introduced into an n-type substrate. Diode 102 is typically mounted on a metal frame 104 which, along with encapsulating material 106, serves as the case for sensor 100. The case is characterized by a case temperature, T.sub.c. As noted, when measuring the temperature of a device, sensor 100 is placed in close proximity to the device of interest, separated by a region characterized by an ambient temperature, T.sub.A.
For sensor 100 of FIG. 3, T.sub.j and T.sub.A are related by: EQU T.sub.j -T.sub.A =.theta..sub.jA *P.sub.j,
where .theta..sub.jA is the thermal resistance between the junction of the diode and the ambient environment, and P.sub.j is the power dissipated at the junction. If the power dissipated at the junction is small, then ##EQU2## Thus, in this situation, the ambient temperature is very close to the junction temperature. This means that the junction temperature can be used as a reasonably good approximation to the ambient temperature. Thus, a junction diode with very small power dissipation can be used to accurately sense the temperature of an ambient environment. Note that this does not address the issue of whether the ambient temperature is an accurate indication of the temperature of a semiconductor device in the ambient region.
As recognized by the inventors of the present invention, the conventional temperature sensing method described with reference to FIGS. 1-3 may not be satisfactory for modern integrated circuits. As devices with higher clock speeds are developed, the accuracy with which the temperature of a device can be determined becomes more important. This is because such devices typically generate more heat than lower clock speed devices. It is important to closely monitor the heat generated, and hence the temperature, to prevent device failures. Thus, knowing the actual device temperature during operation provides assistance in assessing the reliability and performance of a device.
FIG. 4 is a diagram showing how the temperature sensor of FIG. 3 is typically used to sense the temperature of an integrated circuit chip, or of a device formed on the chip. In the Figure, IC.sub.1 is the integrated circuit for which the temperature is to be measured. Integrated circuit IC.sub.1 is shown mounted on a printed circuit board. An example of such a circuit would be a microprocessor. IC.sub.2 is integrated circuit temperature sensor 100 of FIG. 3 which is used to measure the ambient temperature of the space underneath IC.sub.1. Since EQU T.sub.j2 -T.sub.A =.theta..sub.j2A P.sub.j2,
and P.sub.j2 is very small, T.sub.j2 .apprxeq.T.sub.A. However, P.sub.j1 (the power dissipation of IC.sub.1) is not necessarily small. In addition, its value is also time dependent. Given the relationship EQU T.sub.j1 -T.sub.A =.theta..sub.j1A P.sub.j1,
it is apparent that measuring only the value of T.sub.A is insufficient to determine the temperature of IC.sub.1 (i.e., T.sub.j1). It is also necessary to know the values of .theta..sub.j1A and P.sub.j1. .theta..sub.j1A may be separately measured and determined for a specific package containing IC.sub.1. However, P.sub.j1 is not a constant and is dependent upon the operating status of IC.sub.1.
During the operation of currently available high speed integrated circuits, P.sub.j1 can change from a very small value (e.g., in the idle mode) to very large values at full speed operation. This is particularly true for microprocessors running at clock speeds of hundreds of MHz. Since the power dissipation in integrated circuits is directly proportional to the clock speed, the value of P.sub.j1 is a term which cannot be reliably estimated or overlooked. This means that the conventional, remote temperature sensing method cannot, in many instances, be used to accurately determine the temperature of high speed devices.
What is desired is an apparatus and method for accurately determining the actual operating temperature of a semiconductor device.